Fifty years ago this July, Électricité de France began sealing off Normandy’s La Rance estuary from the sea. After three years of work, the world’s first large-scale tidal power plant was born. The station operates still, generating up to 240 megawatts of renewable power as the twice-daily tides force water in and out of the estuary through the hydroturbines seated within its 750-meter-long seawall.

But the three years of construction were tideless, which devastated La Rance’s ecosystem, killing off nearly all of its marine flora and fauna; it would take another decade for the estuary to bounce back. Due in part to that ecological hangover, La Rance would remain the only tidal station of its scale for nearly five decades. Recently, however, capturing tidal energy using impoundments—reservoirs formed by dams—is making a comeback. In 2011 South Korean officials turned on a 254-MW “barrage” style plant akin to La Rance’s, and further installations are in development in South Korea and elsewhere, including the United Kingdom and China.

Renewed interest in tidal impoundments is being driven by the global push for renewable energy. Tidal power is an especially attractive option given its predictability, and impoundments are proven technology. Whereas freestanding underwater turbines designed to capture tidal currents are still being tested at the megawatt scale, La Rance has delivered 500 to 600 gigawatt-hours annually since 1967, reliably generating power for more than 15 hours per day.

Of course, tidal stations also have a proven ecological impact even during operation. Tides get delayed as they flow through the turbines, altering currents and sea levels on both sides of the seawall.

South Korea’s two-year-old tidal station was ecologically justified as a solution to a more severe problem. Its turbines were a retrofit for the 11-kilometer-long seawall whose completion in 1994 impounded a body of water in the northwest known as Lake Sihwa. After industrial effluent polluted the lake, the seawall was reopened to flush the pollution out. As a legal expert explained it several years ago, generating power while cleaning the lake enabled the government to—as Koreans say—“catch two pigeons with one bean.”

Success, however, created an appetite for tidal projects in relatively pristine areas of South Korea. Contracts signed last year, for example, would put an 810-MW plant at Ganghwa Island, which abuts smaller islands where critically endangered black-faced spoonbill seabirds feed and breed. Yekang Ko, a professor of urban and public affairs at the University of Texas, says the plant could flood the spoonbill’s habitat. Similar concerns surround a high-profile proposal in the U.K. to build a barrage across the Severn estuary in Wales.

In contrast, a 250-MW tidal plant proposed by Cheltenham, England–based Tidal Lagoon Power for Wales’s Swansea Bay is—at least so far—conflict-free. As the firm’s name implies, the plant’s seawalls would create an artificial lagoon out in the bay rather than isolating an estuary. With just a relatively small link to the shore, the lagoon design should have limited impact on the most sensitive intertidal zones, says Gareth Clubb, Wales director for Friends of the Earth. “We’ve been strong supporters of the Swansea tidal lagoon and resolute opponents of the Severn barrage,” says Clubb.

In fact, Clubb says, lagoon-style plants could buffer the U.K.’s Atlantic exposure against the rising seas and intensifying storms predicted by climate change modelers. And since tides arrive at different times along the coast, the ensemble would deliver a relatively steady flow of renewable power.

Ton Fijen, Tidal Lagoon Power’s technical director, predicts that the cost per megawatt-hour of the Swansea project’s power will be in line with the cost of onshore wind power—currently the cheapest source of renewable energy. If he is right, and environmentalists remain supportive, tidal power could really take off without taking the world’s estuaries with it.

This article originally appeared in print as “Tidal Energy Turns a Corner.”